In a typical cellular system, also referred to as a wireless communications network, wireless terminals, also known as mobile stations or user equipments communicate via a Radio Access Network, RAN, to one or more core networks. The radio access network may comprise access points, AP, or base stations, BS that communicate with the user equipments by means of radio signals and provide access to the core network.
The Third Generation Partnership Project, 3GPP, has established a plurality of generations of mobile communication standards. The Universal Mobile Telecommunications System, UMTS, is a third generation mobile communication system, which evolved from the Global System for Mobile Communications, GSM, to provide mobile communication services based on Wideband Code Division Multiple Access, WCDMA, access technology. Long-Term Evolution, LTE, often being referred to as fourth generation, has been specified to increase the capacity and speed using a different radio interface together with core network improvements. The standard is specified in several releases starting with Release 8 document series, and comprising enhancements described in further releases.
In cellular communication systems, downlink, DL, pilots or reference signals (RS) of predefined and known characteristics are regularly transmitted by the infrastructure access points or base stations of the radio access network to the user equipments. The reference signals are used (e.g. measured) by both idle and active user equipments, e.g. for the purpose of mobility measurements, cell association, as reference for channel state information, CSI, estimation and data demodulation, or supporting channel state dependent scheduling algorithms (the exact usage may be system dependent). CSI refers to known channel properties of a communication link.
In the following, the current technique will be described for the example of LTE. According to LTE specifications, some of the reference signals are called cell specific reference signals, CRS, that have a predefined pattern covering the entire frequency band, and are transmitted four times per millisecond (assuming two antenna ports).
According to 3GPP Technical Specification 36.213, current version 12.5.0, in the following being referred to as TS 36.213, the UE shall perform a periodic and/or an aperiodic reporting of channel state indicators. This information may be used by the radio access network for scheduling decisions (e.g. comprising a selection of a modulation and coding scheme, MCS, to be used by the UE for transmitting a certain transport block, and a resource block, RB, allocation to the UE) to ensure an efficient usage of radio resources.
The CSI feedback transmitted by the UE in the uplink can be regarded as an implicit indication of the data rate which can be supported by the wireless channel, taking into account the prevailing or predicted Signal to Interference plus Noise Ratio, SINR, level and characteristics of the UE receiver.
According to 3GPP TS 36.213, section 7.2, the CSI to be reported by the UE comprises a so-called Channel Quality Indicator, CQI, and may comprise further indicators such as a so-called Pre-coding Matrix Indicator, PMI, and a so-called Rank Indicator, RI.
According to TS 36.213, section 7.2.3, the UE shall report to the radio terminating node of radio access network, the highest wideband CQI value within a range of suitable CQI values matching to an instantaneous radio condition experienced by the UE, given the receiver capabilities of the UE, resulting into a block error rate, BLER, equal or below 10%. The BLER therein represents a ratio of a number of erroneous data blocks and a corresponding total number of received data blocks. As currently defined in above-cited TS 36.213, there are 16 CQI values from 0 to 15 each associated to a certain efficiency, wherein the code rate and hence the efficiently increases with the value.
For certain transmission modes, the precoding matrix indicator, PMI, received from the UE may be used for channel dependent codebook based precoding.
For spatial multiplexing, the UE determines a RI corresponding to the number of useful transmission layers.
The appropriate access point or base station, also being referred to as eNodeB or eNB in the context of LTE, selects different CSI or CQI feedback modes that trade off improved downlink modulation and coding scheme (MCS) selection against the uplink overhead that CQI feedback implies. According to the preceding discussion, the CQI feedback is derived from the downlink reference signals, based on the UE measurements that allow the UE to estimate the highest MCS (rate) that the UE expects to be able to decode with some predefined bit error rate or block error rate, BLER, target. For example, a UE with advanced receiver capabilities and interference cancellation capabilities may report a higher MCS value than a UE with a low complexity receiver structure under similar interference and expected SINR conditions. RI and PMI feedback from the UE may be used by the eNB to select an appropriate rank and pre-coding for Multiple Input Multiple Output, MIMO, operations of the UE.
Other cellular and wireless technologies (e.g. wideband code division multiple access (WCDMA), high speed packet access, WiMax) typically provide some (similar) type of pilot or reference signals (for example in the form of pilot channels as in WCDMA) to support mobility measurements, channel state dependent algorithms or demodulation of control and data information.
Classical wireless systems are designed on the premise of half-duplex (HD) communication that does not allow a simultaneous transmission and reception of radio signals on the same frequency channel. Examples on HD transmission and reception schemes include half duplex frequency division duplexing (HD FDD) and time division duplexing (TDD) that enable separating the transmitted and received signals at a radio transceiver either in frequency or in time or in both. Full duplex frequency division duplexing (FD FDD) enables simultaneous transmission and reception of radio signals but the transmission and reception of radio signals take place on different carrier frequencies.
In contrast, full-duplex (FD) communication enables simultaneous transmission and reception of radio signals on the same carrier frequency.
FD communication systems face the problem of self-interference, SI, in a way that the received signal from peer transmitter is disturbed by the signal sent by the own transmitter. The caused SI thus depends on the own transmit power; in case that the device is a UE, the SI is a function of the UL transmit power.
Recently efforts have taken to overcome the basic assumption that full duplex communication may not be practically viable due to the large SI caused by a radio transmitter at the radio receiver. Thereto, full-duplex capable devices may be equipped with analog cancellation circuitry operating at radio frequency and/or with digital cancellation circuitry operating at baseband frequency. However, the remaining SI after cancellation may still be significant especially in situations wherein the transmit power is rather high compared to the receive power at the device.